These are millipedes in the species Pachydesmus crassicutis that Brent and I collected from Homochitto National Forest in Mississippi last week. There’s one male and one female in the picture, and if you asked me which one is the male, I would say that the bottom one is. I’d say that because in most Arthropoda (including millipedes) the female tends to have a larger body than the male. The common reason for this is larger females pass on more offspring, which are also larger, and this tends to be favored. (More eggs can be packed into a larger millipede.) Anyways, in the millipedes above, there’s distinct sexual size dimorphism, which means there’s a difference in size between genders. However in this case, the male is quite a bit larger than the female, and I noticed this difference consistently in about 10 females and 10 males encountered while collecting that night. This is an unusual phenomenon for me to encounter because male millipedes are always smaller than females (for example these Sigmoria nantahalae millipedes). It isn’t clear why this is the case, but looking at the function of big males in other arthropods it’s usually associated with male-to-male competition for female mates. A larger male tends to outcompete a smaller male in contests for access to females. Interestingly, larger males are more common among vertebrates, for example human males are about 1.1-fold bigger than females and gorillas about 1.5-fold bigger.

Movie of Pachydesmus crassicutis walking (HD, big file 30 MB)

Craig Stillwell and Goggy Davidowitz from the University of Arizona and colleagues wrote an excellent review of sexual size dimorphism and how size differences are affected by physiological and developmental processes: Sex differences in phenotypic plasticity affect variation in sexual size dimorphism in insects: from physiology to evolution. Annual Review of Entomology, 55, 227-245.